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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => AC Power and Grounding => Topic started by: Ed Hall on September 27, 2017, 12:06:57 pm

Title: Could this happen in a properly wired outlet?
Post by: Ed Hall on September 27, 2017, 12:06:57 pm
A friend had a kitchen outlet arc and trip the GFCI. The cover around the outlet is metal and should be grounded. The arc happened when a metal tape measure brushed in front of the outlet. It looks like the arc was between the hot and cover. 
Could a metal tape in contact with the grounded cover and in front of the hot receptacle cause the arc? I would think that shouldn't happen. 

FYI, she asked me what happened and my reply was it wanted you to call an electrician.

(https://uploads.tapatalk-cdn.com/20170927/31b48a4219d14f2c28f5fb3c3f9b8d84.jpg)
Title: Re: Could this happen in a properly wired outlet?
Post by: David Allred on September 27, 2017, 12:20:34 pm
Were the plugged in cord's blades flashed or arced?  Is the soot coming from the outlet or the plug?  visually, it look like what I would expect from a plug blowing up.
Title: Re: Could this happen in a properly wired outlet?
Post by: Stephen Swaffer on September 27, 2017, 12:22:03 pm
Was there anything plugged into the outlet?  This looks more like a metal tape measure slipped behind a partially plugged in cord and contacted the hot and the neutral-that would most certainly cause this, GFCI or not. 

Otherwise it seems unlikely-the hot contacts are typically recessed .100 or better behind the face-far too far for 120 V to jump without actually sticking something into the receptacle.
Title: Re: Could this happen in a properly wired outlet?
Post by: Jerome Malsack on September 27, 2017, 01:07:49 pm
We might also consider the size of the tip of the tape measure reaching into the Hot side.  One would need to see the size of the tape measure and the end to see if, the end fits in the outlet.  Larger 1/2 inch size may not fit and have a problem where as the smaller sizes may fit in the open slot. 
Title: Re: Could this happen in a properly wired outlet?
Post by: David Allred on September 27, 2017, 01:23:12 pm
I read the tape measure part, but did not connect that it was a spring-wound measuring device.  Duh.

There is no arc pitting on the SS cover.  I don't think ii touched it.  Had to be a plug in the outlet.  The picture implies that the tape diverted the flash downward from the resting points on the blades.
Title: Re: Could this happen in a properly wired outlet?
Post by: John Roberts {JR} on September 27, 2017, 02:10:54 pm
A friend had a kitchen outlet arc and trip the GFCI. The cover around the outlet is metal and should be grounded. The arc happened when a metal tape measure brushed in front of the outlet. It looks like the arc was between the hot and cover. 
Could a metal tape in contact with the grounded cover and in front of the hot receptacle cause the arc? I would think that shouldn't happen. 

FYI, she asked me what happened and my reply was it wanted you to call an electrician.

(https://uploads.tapatalk-cdn.com/20170927/31b48a4219d14f2c28f5fb3c3f9b8d84.jpg)
If a metal tape measure shorts between hot and safety ground that will trip the GFCI as loop current is diverted to ground instead of returning into neutral.

The voltage of only 120VAC is not enough to arc across any appreciable distance without an ionized path, after the first arc is struck the pyrotechnics can last a little longer but the GFCI will trip in mSec.

It seems like a freak occurrence for a loose tape measure end to make electrical contact with the line socket lead, "and" faceplate ground but the evidence suggests it did happen. And the GFCI worked to protect the meat puppets.

JR
Title: Re: Could this happen in a properly wired outlet?
Post by: Jonathan Johnson on September 27, 2017, 03:39:01 pm
I'll jump in and play the guessing game, too.

Most of these measuring tapes have paint and a mylar coating that provides a minimal amount of electrical insulation, but the edge of the tape sees a lot of abrasion so may have exposed metal.

I'll guess that something was plugged in, and someone was measuring the length of the wall across or above the outlet.

The metal rule of the measuring tape slipped down between the plug and the face of the receptacle. The edge of the tape, which probably had bare metal, contacted the hot and neutral prongs, resulting in an arc flash. (The sharp line across the top of the holes suggests this.) This alone would not have tripped the GFCI, as the current between hot and neutral would still be balanced. Also, if the initial short was to ground, the GFCI may have tripped before the current was sufficient to create an arc flash.

However, the initial arc flash melted the coating from the rule, exposing more bare metal. After the initial arc flash, continuing motion of the tape (or reaction of personnel) caused the bare metal to contact the cover plate while the edge of the rule was still contacting either the hot or neutral prongs, resulting in a current imbalance between hot and neutral that tripped the GFCI.

Yes, it could happen in a properly wired outlet. Something similar happened to me when I was about 6 years old. I was playing, hanging metal coat hangers on an electrical cord that was draped from a receptacle to a table lamp. I then hung a coat hanger on the plug, where it promptly fell into the gap between the plug and the face of the receptacle, causing a short circuit, an arc flash, and a tripped breaker. (This was not a GFCI protected circuit.) I was a fortunate child, and suffered no injury.

(On deeper analysis, the blade of the tape contacted the neutral prong or cover plate first. We can tell this, because the carbonation radiates from the contact point on the top of the hot prong. If it contacted the hot first, the carbonation would radiate from the second point of contact when the circuit was completed. This also explains where there is no pitting on the cover plate; if indeed it was a short between the cover plate and the hot prong, contacting the cover plate first would ensure that the arc would form at the second point of contact, the hot prong.)
Title: Re: Could this happen in a properly wired outlet?
Post by: Ed Hall on September 27, 2017, 04:10:57 pm
She claimed the "metal tape measure brushed up against it"  She was measuring and the contact was incidental. I'm waiting to hear back to verify that nothing was plugged in at the time.

Yes, in aware it can arc under the proper circumstances, like a "U" shaped piece of plumbing solder inserted into the hot and neutral with a pair or pliers. Don't ask!?!? I didn't think a metal tape that's not inserted at all would be a risk for an arc flash.
Title: Re: Could this happen in a properly wired outlet?
Post by: Keith Broughton on September 27, 2017, 05:11:07 pm
I'm not buying the tape measure scenario.
How could you possibly get the metal tip of a tape into the receptacle far enough AND keep contact with the plate to get that kind of spark damage?
The plastic on the receptacle "hot" slot is a bit melted. That's a lot of spark damage!
Where is the damage on the plate that would correspond to the contact location for a direct hot/ground short?
If the tape crossed the neutral and hot of a partially exposed plug, the damage would look different, IMHO.
Looks like something was plugged in and had a hot/ground short.
Makes me go Hmmmmmmmmm.......
Title: Re: Could this happen in a properly wired outlet?
Post by: Stephen Swaffer on September 27, 2017, 07:39:46 pm
Assuming the GFCI was functioning properly, a hot-ground short should have tripped the GFCI-and typically these react at least 5-6 times faster than a circuit breaker. It would surprise me to see this much arcing from a GFCI protected fault-though I still find myself surprised quite often.
Title: Re: Could this happen in a properly wired outlet?
Post by: Paul Miller on September 28, 2017, 01:44:54 am
For reference, this is what it looks like after a very thin outlet cover hinge clip falls on top of exposed hot and neutral prongs. The result was a tripped circuit breaker, a vaporized hinge clip, and the visible charring. The damage in this and the OP's picture look very similar to me, which suggests the tape measure falling across the exposed prongs scenario.

We could lessen the likelihood of such accidents by simply installing receptacles upside down, so that the ground prong would be the first thing struck by a falling metal object. Surely I'm not the first one to realize this.
Title: Re: Could this happen in a properly wired outlet?
Post by: Lyle Williams on September 28, 2017, 05:59:44 am
The logic is when pulled downwards (most common direction) earth is the last thing to disconnect.

Some countries do things differently.  Australian and Chinese 3-pin plugs are pretty well interchangeable, but the Chinese install sockets with the earth pin upwards.

In a kitchen it may be clear what sort of appliance should have been plugged into a given socket.  Find the appliance and look at the plug.  Or find the missing item (eg kitchen with no kettle or brand new kettle) to understand what probably happenned.
Title: Re: Could this happen in a properly wired outlet?
Post by: Rob Spence on September 28, 2017, 11:42:49 am
The logic is when pulled downwards (most common direction) earth is the last thing to disconnect.

Some countries do things differently.  Australian and Chinese 3-pin plugs are pretty well interchangeable, but the Chinese install sockets with the earth pin upwards.

In a kitchen it may be clear what sort of appliance should have been plugged into a given socket.  Find the appliance and look at the plug.  Or find the missing item (eg kitchen with no kettle or brand new kettle) to understand what probably happenned.

In many commercial installs, the receptacle is installed ground pin up. Most residential installs have it down.

Lots of rationals as to why.



Sent from my iPad using Tapatalk Pro
Title: Re: Could this happen in a properly wired outlet?
Post by: Stephen Swaffer on September 28, 2017, 12:36:04 pm
In many commercial installs, the receptacle is installed ground pin up. Most residential installs have it down.

Lots of rationals as to why.



Sent from my iPad using Tapatalk Pro

And both methods are correct-until a given receptacle is involved in a fault where the other method would have prevented the issue then the guy that installed that receptacle was an idiot :o

Something configured like DIN connector or XLR with the ground shell completely surrounding the pins and longer would be the safest-but we only use connectors like that on low voltage stuff.  Note I am NOT suggesting using those particular connectors for mains-just alluding to the design style.
Title: Re: Could this happen in a properly wired outlet?
Post by: Chris Hindle on September 28, 2017, 01:41:35 pm
She claimed the "metal tape measure brushed up against it"  She was measuring and the contact was incidental. I'm waiting to hear back to verify that nothing was plugged in at the time.

Yes, in aware it can arc under the proper circumstances, like a "U" shaped piece of plumbing solder inserted into the hot and neutral with a pair or pliers. Don't ask!?!? I didn't think a metal tape that's not inserted at all would be a risk for an arc flash.

My vote is a 2-prong appliance was plugged in, almost completely, and the tape measure brushed across the hot and neutral from the bottom side.
That would make a nice pffft. Bet you can't read the tape......
I don't believe the cover played a part at all.
Chris.
Title: Re: Could this happen in a properly wired outlet?
Post by: David Allred on September 28, 2017, 01:51:23 pm
My vote is a 2-prong appliance was plugged in, almost completely, and the tape measure brushed across the hot and neutral from the bottom side.
That would make a nice pffft. Bet you can't read the tape......
I don't believe the cover played a part at all.
Chris.

Only if the pic is upside down. :)
Title: Re: Could this happen in a properly wired outlet?
Post by: Jonathan Johnson on September 28, 2017, 04:02:44 pm
In many commercial installs, the receptacle is installed ground pin up. Most residential installs have it down.

Lots of rationals as to why.

The two main rationales are:

Men usually pick option one or don't care.

Women usually pick option two.

Women almost always win the debate. (Except in commercial installs where the women often don't have a say.)
Title: Re: Could this happen in a properly wired outlet?
Post by: Ed Hall on September 28, 2017, 04:14:22 pm

(https://uploads.tapatalk-cdn.com/20170927/31b48a4219d14f2c28f5fb3c3f9b8d84.jpg)

So I spoke with her again. She doesn't remember(?) if a plug was in the receptacle or not.  Looking at it again I believe there was. The carbon looks like it originated from the top of the hot slot. It radiates out fairly evenly below and to the sides of the slot. There is a relatively sharp line above the slot where the carbon stops. Presumably masked by the metal tape.  There is no carbon to the left of the neutral slot, but it extends below the slot to the edge of the receptacle. This looks like a similar masking of the flash by the neutral prong. I'm not sure how the flash and resulting carbon would stop at the slot if nothing was there.
Title: Re: Could this happen in a properly wired outlet?
Post by: Keith Broughton on September 28, 2017, 06:10:33 pm
" Presumably masked by the metal tape."
You make a good case.
Title: Re: Could this happen in a properly wired outlet?
Post by: John Roberts {JR} on September 29, 2017, 09:38:46 am
My vote is a 2-prong appliance was plugged in, almost completely, and the tape measure brushed across the hot and neutral from the bottom side.
That would make a nice pffft. Bet you can't read the tape......
I don't believe the cover played a part at all.
Chris.
That would not automatically trip the GFCI.

JR
Title: Re: Could this happen in a properly wired outlet?
Post by: Marc Sibilia on September 29, 2017, 10:12:50 am
That would not automatically trip the GFCI.

JR

The arc could have started from the hot to the neutral, but with the conductive plasma filling the area between the plug and the receptacle face, it is highly likely that some current sharing would occur to the ground pin, and that would trip the GFCI.
Title: Re: Could this happen in a properly wired outlet?
Post by: Dennis Wiggins on September 29, 2017, 08:37:11 pm
My vote is a 2-prong appliance was plugged in, almost completely, and the tape measure brushed across the hot and neutral from the bottom side.
That would make a nice pffft. Bet you can't read the tape......
I don't believe the cover played a part at all.
Chris.

I'll second this... OK. I'll spill my beans.  I once 'cleverly' hung a key chain by its metal key ring around a counter top night light body (so I would always know where the key was).   ;)

Guess what happened a year later when I unplugged the nightlight to replace the bulb?  Exactly  as as shown in the picture. The exception being my cover plate was plastic.

-Dennis
Title: Re: Could this happen in a properly wired outlet?
Post by: Mike Sokol on September 30, 2017, 10:48:30 am
That would not automatically trip the GFCI.

JR

Yeah, a hot-to-neutral short won't trip the GFCI since there's no current imbalance. So a hot-to-ground short will trip it immediately once you get over 5mA current. And interestingly, a ground-to-neutral short can trip one since if you're on a branch circuit with a normal voltage drop on a neutral, you can get up to 3 or 4 volts between neutral and ground and still be within normal limits. So a short between the neutral and ground will create a fault current, and it's certainly enough to trip any GFCI due to the imbalance. 
Title: Re: Could this happen in a properly wired outlet?
Post by: John Roberts {JR} on September 30, 2017, 12:38:12 pm
Yeah, a hot-to-neutral short won't trip the GFCI since there's no current imbalance. So a hot-to-ground short will trip it immediately once you get over 5mA current. And interestingly, a ground-to-neutral short can trip one since if you're on a branch circuit with a normal voltage drop on a neutral, you can get up to 3 or 4 volts between neutral and ground and still be within normal limits. So a short between the neutral and ground will create a fault current, and it's certainly enough to trip any GFCI due to the imbalance.
I just had another spurious GFCI trip on the GFCI outlet protecting my dishwasher and washing machine. This is a little awkward because the outlet for the dishwasher is out in the laundry room so I can't see the red light from inside my kitchen.

Connecting the dots, we had a power outage between when the GFCI outlet was last working and not, so apparently it tripped (or didn't set) when power was restored. I have several GFCI outlets around the house and only this one was affected by the power outage. This may be the second time this happened, but the first time I connected it to the power interruption.

JR

[edit- I had another spurious trip in the GFCI my dishwasher/washing machine is plugged into. This time it tripped while the washing machine was running. Since the washing machine is probably 20 YO, who knows? Finished the wash after being reset and didn't trip next time I did laundry. I am past due for new appliances but why fix it if it isn't broken. [/edit]   
Title: Re: Could this happen in a properly wired outlet?
Post by: Ken Cross on October 22, 2017, 02:57:57 pm
Assuming the GFCI was functioning properly, a hot-ground short should have tripped the GFCI-and typically these react at least 5-6 times faster than a circuit breaker. It would surprise me to see this much arcing from a GFCI protected fault-though I still find myself surprised quite often.

The tape measure senerio listed happened exactly that way in my garage a few years ago, and it was me who was holding the tape. Yes it was a GFI outlet and yes I had my band saw plugged in. the tape contacted the top of the band saw contacts. In my case (plastic box and cover) my hand on the tape and feet on the concrete floor tripped the GFCI. The result also looked the same. The GFCI saved me from feeling the shock. The arc still got my attention in a hurry.

Ken
Title: Re: Could this happen in a properly wired outlet?
Post by: Jeff Hague on October 26, 2017, 04:11:49 pm


We could lessen the likelihood of such accidents by simply installing receptacles upside down, so that the ground prong would be the first thing struck by a falling metal object. Surely I'm not the first one to realize this.

I see exactly this in a lot of new construction lately - the receptacles are "upside down". But it does make a lot of sense.
Jeff
Title: Re: Could this happen in a properly wired outlet?
Post by: Steve M Smith on October 27, 2017, 03:10:51 am
We could lessen the likelihood of such accidents by simply installing receptacles upside down, so that the ground prong would be the first thing struck by a falling metal object. Surely I'm not the first one to realize this.

Or you could reduce it to zero by having safe plugs like we do in the UK.

(http://cpc.farnell.com/productimages/standard/en_GB/1718015-40.jpg)


Steve.
Title: Re: Could this happen in a properly wired outlet?
Post by: Jonathan Johnson on October 27, 2017, 02:33:36 pm
Or you could reduce it to zero by having safe plugs like we do in the UK.

(http://cpc.farnell.com/productimages/standard/en_GB/1718015-40.jpg)



Sure, but most AHJ would say "I'm not signing off; that's not the way we do it here."
Title: Re: Could this happen in a properly wired outlet?
Post by: Mike Sokol on October 27, 2017, 03:43:42 pm
Sure, but most AHJ would say "I'm not signing off; that's not the way we do it here."

I've used a lot of Ceeform connectors in systems going out on international tours, and they makes hookup a lot easier and safer as well. But we're stuck with what we have here... :-[
Title: Re: Could this happen in a properly wired outlet?
Post by: frank kayser on October 28, 2017, 09:48:49 pm
I just had another spurious GFCI trip on the GFCI outlet protecting my dishwasher and washing machine. This is a little awkward because the outlet for the dishwasher is out in the laundry room so I can't see the red light from inside my kitchen.

Connecting the dots, we had a power outage between when the GFCI outlet was last working and not, so apparently it tripped (or didn't set) when power was restored. I have several GFCI outlets around the house and only this one was affected by the power outage. This may be the second time this happened, but the first time I connected it to the power interruption.

JR

[edit- I had another spurious trip in the GFCI my dishwasher/washing machine is plugged into. This time it tripped while the washing machine was running. Since the washing machine is probably 20 YO, who knows? Finished the wash after being reset and didn't trip next time I did laundry. I am past due for new appliances but why fix it if it isn't broken. [/edit]   


JR... Are you ever going to rewire that death trap of a house, or do you just like having a lab nearby?  ;)
Title: Re: Could this happen in a properly wired outlet?
Post by: John Roberts {JR} on October 29, 2017, 09:47:21 am

JR... Are you ever going to rewire that death trap of a house, or do you just like having a lab nearby?  ;)
Rewiring this house would be throwing good money after bad***... I just tested my smoke alarm battery when the heat came on for the first time this season.... no visible smoke, but enough to wake up the smoke alarm (pretty impressive actually).

I did keep the outlet by my test bench reversed polarity because it made testing my OD-1 easier....(no ground BTW).

I ran an additional separate ground wire from my panel to the outlet in my kitchen next to the sink, and to the outlet in my laundry room where the washing machine and dishwasher plug in (probably not to code). I grounded my new hot water heater and plumbing too (I won't get fooled again).

My experience with trippy GFCI outlets does not seem to be wiring related (or load as far as I can tell), just unstable operation.

I wonder how regular homeowners deal with the shorter half-life of modern (smart) outlets, or maybe they buy better than the cheapest GFCI they can find like I did?

For somebody paying an electrician real money to replace trippy GFCIs that would get old very fast..

JR

*** I am actually in the final stages of throwing some fresh paint on the shack so it looks better than it has in years. It was looking bad enough that it bothered me and that is saying something. When I'm finished probably $100 for paint and rollers.